An application of a nitride semiconductor to a high-withstand voltage and high-power semiconductor device has been studied by using characteristics such as high saturation electron speed and a wide band gap thereof. For example, a band gap of GaN being the nitride semiconductor is 3.4 eV, which is larger than the band gap (1.1 eV) of Si and the band gap (1.4 eV) of GaAs, and it has high breakdown electric field intensity. Accordingly, GaN is extremely expectable as a material of a power semiconductor device capable of high voltage operation and high-power.
Many reports have been made as for a field-effect transistor, in particular, a High Electron Mobility Transistor (HEMI) as the semiconductor device using the nitride semiconductor. For example, in a GaN based HEMI (GaN-HEMT), an AlGaN/GaN.HEMT in which GaN is used as an electron transit layer and AlGaN is used as an electron supply layer is focused. In the AlGaN/GaN.HEMT, distortion resulting from a lattice constant difference between GaN and AlGaN occurs at AlGaN. High-concentration two-dimensional electron gas (2DEG) is obtained by piezoelectric polarization generated by the distortion and spontaneous polarization of AlGaN. Accordingly, it is expected as a high-withstand voltage power device for a switch element, an electric vehicle, and so on.
It is an important issue common to a semiconductor device using a nitride semiconductor to reduce an off-leakage current. GaN is easy to turn into n-type resulting from crystal defect, mixture of impurities or the like, and it is a problem in which leakage of current occurs via a part of a channel region of GaN being a buffer layer or the electron transit layer. A method in which an impurity such as Fe is doped when the buffer layer grows to make it high resistance is cited as a method to reduce the off-leakage current. However, the buffer layer in a crystal growth is extremely important, and there is a problem in which crystallinity of the electron transit layer, the electron supply layer, and so on is deteriorated, and deterioration of device characteristics is incurred by making the buffer layer into an impurity doping layer.    [Patent Document 1] Japanese Laid-open Patent Publication No. 2007-251144